Remove invalid use of null references (undefined behaviour)

Enable the relevant compiler warning as an error, if supported
(-Werror=undefined-bool-conversion).

http://www.gotw.ca/conv/002.htm
http://stackoverflow.com/questions/2165078/a-reference-can-not-be-null-or-it-can-be-null

(cherry picked from commit f2911ac)
(cherry picked from commit 5a50b98)

lib/common/Timer.cpp

@@ -151,7 +151,6 @@ void Timers::AssertInitialised()
 void Timers::Add(Timer& rTimer)
 {
 	ASSERT(spTimers);
-	ASSERT(&rTimer);
 	BOX_TRACE(TIMER_ID_OF(rTimer) " added to global queue, rescheduling");
 	spTimers->push_back(&rTimer);
 	Reschedule();
@@ -168,8 +167,6 @@ void Timers::Add(Timer& rTimer)
 // --------------------------------------------------------------------------
 void Timers::Remove(Timer& rTimer)
 {
-	ASSERT(&rTimer);
-
 	if(!spTimers)
 	{
 		BOX_WARNING(TIMER_ID_OF(rTimer) " was still active after "

test/common/testcommon.cpp

@@ -312,10 +312,11 @@ int test(int argc, const char *argv[])
 	// Check that using timer methods without initialisation
 	// throws an assertion failure. Can only do this in debug mode
 	#ifndef BOX_RELEASE_BUILD
-		TEST_CHECK_THROWS(Timers::Add(*(Timer*)NULL), 
-			CommonException, AssertFailed);
-		TEST_CHECK_THROWS(Timers::Remove(*(Timer*)NULL), 
-			CommonException, AssertFailed);
+	{
+		Timer tim(0, "tim");
+		TEST_CHECK_THROWS(Timers::Add(tim), CommonException, AssertFailed);
+		Timers::Remove(tim);
+	}
 	#endif
 
 	// TEST_CHECK_THROWS(Timers::Signal(), CommonException, AssertFailed);